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A BASIC IMMERSION FIRETUBE FLOWNEX MODEL This case study demonstrates the implementation of a basic immersion firetube model in Flownex and presents natural draft and forced draft examples. OIL AND GAS INDUSTRY

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Page 1: A BASIC IMMERSION FIRETUBE FLOWNEX MODEL Immersion... · A BASIC IMMERSION FIRETUBE FLOWNEX MODEL This case study demonstrates the implementation of a basic immersion firetube model

A BASIC IMMERSION

FIRETUBE FLOWNEX

MODEL This case study demonstrates the implementation of a basic immersion firetube

model in Flownex and presents natural draft and forced draft examples.

OIL AND GAS INDUSTRY

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Challenge:

The main challenge is to model an immersion firetube in Flownex. Immersion firetubes are widely

used in industry, most commonly in indirect heating applications where either gas or oil burners

are used as a heat source.

Benefits:

Flownex allows the user to model combustion, heat transfer and fluid flow processes in an elegant

and easy to understand way.

Solution:

Using Flownex’s compound component and scripting capabilities, a simple immersion firetube

model has been developed and is presented in this case study. Furthermore, examples of natural

draft and forced draft design cases are presented.

OIL AND GAS INDUSTRY

“Flownex has the unique ability to simultaneously model combustion, heat transfer

and fluid mechanics problems. This capability makes Flownex the ideal tool to

design and analyze immersion firetube heat transfer processes.”

Hannes van der Walt

Principal Thermal Engineer

Gasco Pty Ltd

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Introduction

Indirect heating processes have been widely used in the oil and

gas and several other industries for many decades. Advantages

of indirect heating include:

The relatively low cost of the equipment.

Separation of the high pressure process fluid from the

heating medium via simple pressure piping.

Relatively high efficiencies.

Low maintenance and running cost.

Reduced heat loss.

Long operational life.

The heating medium may be water, water-glycol, salt, steam and air for indirect heaters, or any

fluid that needs to be heated directly. Water is possibly the most common medium due to its low

cost.

The Immersion Firetube

Heating processes may be best explained in terms of a heat balance diagram. Energy is supplied

by combusting a fuel and is referred to as the gross heat input when specified in terms of higher

heating value (HHV) as is typical in natural gas burner applications. The largest loss of heat is likely

to be the hot flue gas that leaves the process via an exhaust stack.

Figure 1: Sankey heat balance diagram (from Eclipse Engineering Guide).

A BASIC IMMERSION FIRETUBE

FLOWNEX MODEL

IN FLOWNEX HEAT TRANSFER MODELS

“It would be much more challenging

to design and analyze an entire

immersion firetube-based heater

system at the level of detail presented

without Flownex as an engineering

design and analysis tool.”

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Smaller amounts of heat may be lost through insulation, radiation and other effects. The difference

between the gross heat input and the sum of all the losses results is the net output (also known as

the heat to load, heat to process or heat duty).

Immersion firetubes, as the name suggests, are tubes or pipes fully immersed in a fluid with a

burner firing into one end. The combustion gases flow through the firetube and leave at the other

end, normally into an exhaust stack. The immersion tube aims to transfer as much heat as possible

to the fluid within the boundaries of inevitable practical constraints. A typical example is shown in

the following figure.

Figure 2: Typical burner and immersion firetube application heating a liquid (from Maxon Series

“67” Tube-O-Flame Bulletin 2200).

In the example above, it is shown that the immersion firetube is a 3-pass unit. However, different

designs are common in industry, each with its own advantages and disadvantages. Some common

immersion firetube layouts are shown in the following figure.

The U-tube design is obviously the simplest and is probably

therefore also the most common. The burner-end straight

leg is called the radiant section since it is subjected to the

direct luminous radiation of the burner flame. The return leg

or legs are not subjected to direct luminous radiation and

mainly receives heat via convection and some gas radiation.

This part of the immersion firetube is known as the

convection section. The W-tube design is possibly the

second-most commonplace. Essentially it has a similar

radiant section to the ordinary U-tube design, but it has 3

times the convection section length and area. The Trident

tube design does not appear to be popular in industry.

Therefore, the Flownex immersion firetube model presented

is limited to U-tube and W-tube designs.

Figure 3: Typical immersion

firetube layouts.

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Immersion Firetube Design

The thermal design considerations of an immersion firetube include the following:

Burner design (natural draft or forced draft operation).

Thermal efficiency.

Heated medium (most commonly water) temperature.

Size (length and diameter).

Burner heat release density (burner duty divided by firetube cross sectional area).

Firetube outer wetted surface heat flux (heat transferred to heated fluid divided by firetube

total outer wetted surface area).

Turn-down operation.

Firetube flue gas pressure loss (especially important for natural draft designs).

Flue gas oxygen content (combustion design – dictated by legislation).

Flue gas temperature (directly related to thermal efficiency).

Flue gas water vapor and SOx acid gas dew point temperatures (important for corrosion

considerations).

Materials of construction (cost, corrosion resistance).

Corrosion allowance (cost, mass).

Mechanical Design

Firetube designs typically consist of Schedule 40 or lighter pipe (6 mm wall thickness is the

recommended minimum to allow for some corrosion). The first straight pass (radiant section)

length should be at least 10 pipe diameters, however in practice it is often significantly longer as it

depends on the flame length of the selected burner. As a rule of thumb, the flame length should

not be longer than 80% of the radiant section, however this is rarely, if ever, a problem.

Natural draft applications are typically based on U-tube designs due to the limited amount of

natural draft available. Forced draft applications are normally based on W-tube designs to take

advantage of the increased efficiency resulting from the longer firetube length. Since forced draft

designs rely on a fan to supply the combustion air flow, firetube pressure loss is not normally an

issue.

Empirical Firetube Thermal Efficiency Correlation

Several factors influence firetube thermal efficiency, however, it was found that the most significant

factors are firetube length and burner duty. Other factors have a much smaller influence. A simple

empirical efficiency correlation based on higher heating value (HHV) was developed by the AGA

Testing Laboratories and published in 1944 in Research Bulletin No. 24 “Research Fundamentals of

Immersion Tube Heating with Gas”:

(

) Equation 1

where is the thermal efficiency of the immersion firetube based on HHV [%].

is the effective immersion firetube length [ft]. The effective length is the physical

centerline length plus 1.1 ft additional for each return bend. Field tests revealed that return

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bends slightly increase the firetube thermal efficiency due to the effects of improved heat

transfer.

is the burner heat release based on HHV [103 BTU/hr].

At first glance the equation seems to suggest a minimum assumed efficiency of 71%, but as will be

shown below, the constant of 71 is simply a unit-dependent constant which could also be written

inside the brackets of Equation 1. The above equation accepts the burner duty in 1000 BTU/hr

units, however if the burner duty was supplied in BTU/hr instead, Equation 1 would be written as:

(

) Equation 2

where is the burner heat release based on HHV [BTU/hr].

Therefore, for engineers in the modern era, this equation could also be written in SI units as

follows:

(

) Equation 3

where is the thermal efficiency of the immersion firetube based on HHV [%]

is the immersion firetube effective length [m]. The effective length is the physical

centerline length plus 0.335 m additional for each return bend.

is the burner heat release based on HHV [W or kW]

is a unit-dependent constant, where

if the burner heat release based on HHV is specified in kW

if the burner heat release based on HHV is specified in W

Figure 4 below was published in the Eclipse Combustion Engineering Guide (1986) Tech Notes

Section 3 Sheet L-1 Immersion Tube Sizing and was generated from Equation 1.

The correlation given by Equation 1 (or Equation 3) is quite simple, easy to implement in a model

and produces reasonable results, however, it appears to have one limitation. For very long

immersion firetubes, Equation 1 will predict efficiencies larger than 100%. This is the case when the

ratio is:

(

) Equation 4

This equation may be simplified to show that efficiencies of 100% and larger will be calculated

when:

√ Equation 5

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Figure 4: Firetube thermal efficiency as a function of effective firetube length and heat transfer

rate.

Figure 5: The maximum “100% Efficient” immersion firetube length as a function of burner duty.

0

20

40

60

80

100

120

140

160

180

0 500 1000 1500 2000 2500 3000

Max

imu

m L

en

gth

[m

]

Burner Duty [kW]

Maximum Immersion Firetube Length

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As shown above, very long immersion firetubes are required to calculate efficiencies of 100% or

above, and therefore the upper limit is not normally an issue. Furthermore, efficiencies are

normally kept below 85% in an effort to avoid condensation of water and sulfur in the firetube,

especially in turn-down operation. Condensation may cause corrosion problems and will be

discussed in more detail below. API 12K recommends minimum stack temperatures of

approximately 120°C for sulfur-free fuels and 150°C to 200°C for fuels containing sulfur. The

exhaust stack flue gas temperature should remain comfortably above the SOx acid dew point

temperature at all times. A margin of at least 20°C during the worst operating case is often

recommended.

Burner Heat Release Density

Natural draft immersion firetubes have relatively large diameters to ensure that the amount of

natural draft created by the exhaust stack is sufficient to drive the flow of atmospheric air into, and

flue gases through the firetube and stack. The recommended firetube diameter is typically

expressed in terms of a heat flux - known as the burner heat release density :

⁄ Equation 6

where is the burner heat release density [W/m2 or kW/m2]

is the firetube internal cross sectional area at the burner-end [m2]

This serves as a rule of thumb to ensure burners function properly under the conditions of limited

available natural draft.

API 12K recommends a maximum burner heat release density of 15000 BTU/hr.in2

(6814 kW/m2) for natural draft burners.

For forced draft burners, much higher burner heat release densities are achieved. For

example, the following table is from Eclipse’s ImmersoJet Design Guide 330:

Figure 6: Eclipse ImmersoJet Burner Capacity Guide.

Using the DN250 (8”) nominal pipe diameter listed and the 2344 kW burner duty

achievable for the Remote Blower option, a burner heat release density of 72600 kW/m2 is

calculated.

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Eclipse Engineering Guide, Tech Notes Section 3 Sheet L-1 provides the following table

relating to maximum burner heat release density values for natural and forced draft

applications:

Table 1: Maximum Burner Heat Release Densities (Eclipse Engineering Guide, Tech Notes

Section 3 Sheet L-1).

Burner System Type

Max Burner Heat

Release Density

[BTU/hr.in2]

Max Burner Heat

Release Density

[kW/m2]

Atmospheric, natural draft, 7' (2.1 m) high stack 7000 - 8000 3180 - 3634

Atmospheric with eductor, 0.2" w.c. (50 Pa) draft 15000 - 18000 6814 - 8177

Atmospheric with eductor, 0.4" w.c. (100 Pa) draft 21000 - 25000 9540 - 11360

Packaged forced draft, low pressure fan 15000 - 35000 6814 - 15900

Sealed nozzle-mix, high pressure blower 30000 - 85000 13628 - 38612

Small bore nozzle-mix 80000 - 180000 36340 - 81767

Firetube Heat Flux

Another important parameter is the firetube heat flux :

⁄ Equation 7

where is the firetube heat flux over its outside wetted surface area [kW/m2]

is the heat transfer to the process fluid [kW]

is the firetube outside wetted surface area [m2]

This parameter is considered important to prevent unwanted boiling of- and/or thermal damage

to the heated fluid in direct contact with the firetube outside surface. As a result, firetube heat flux

recommended limits depend on the application. The following guidelines are found in the open

literature:

API 12K recommends a firetube heat flux upper limit of 12000 BTU/hr.ft2 (37.8 kW/m2) for

water-glycol heating applications. This limit does not apply for pure water heating

applications.

The GPSA Engineering Data Book presents the following recommended average firetube

heat flux ranges for different applications:

Table 2: GPSA Engineering Data Book Recommended Average Firetube Heat Flux.

Heating Application Firetube Heat Flux

kW/m2

Heating

Application

Firetube Heat Flux

kW/m2

Water 32-41 Molten Salt 47-57

50% Ethylene Glycol 25-32 TEG Reboiler 19-25

Low Pressure Steam 47-57 Amine Reboiler 21-32

Hot Oil 19-25

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The above figures are likely to be quite conservative. For example, pure water heaters are

known to have successfully operated at average firetube heat fluxes as high as 66 kW/m2.

Firetube First Turn-Around Temperature

The flue gas temperature at the end of the firetube radiant section, i.e. at the first turn-around

U-bend depends on numerous factors, most of which are beyond the scope of this discussion. The

Petroleum Technology Alliance Canada (PTAC) produced an excellent report in August 2005 titled

“Improved Immersion Firetube Heater Efficiency Project” in which immersion firetube performance

was analyzed in depth. Several burners were bench-tested in detail albeit these were relatively

small units. Nevertheless, that study showed that the flue gas temperature at the first turn-around

typically varied between 400°C and 800°C depending on the burner duty. For the burners tested,

the higher temperatures occurred during maximum burner duty whereas the lower temperatures

were achieved during 4:1 turn-down operation. Since burners are often selected to have 15% to

20% excess capacity, a good estimate of the first turn-around temperature at 100% firetube duty

(80% burner capacity) would be 700°C.

For forced draft units, the mechanisms for heat transfer may be influenced by the significantly

higher flue gas velocities. Two main influences may be identified:

1. Convection heat transfer will be higher due to the increased velocities.

2. Radiation heat transfer may also be influenced. Firstly; forced draft systems will likely have

different flame shapes (flame length and diameter) which will impact on the direct

luminous flame radiation. Secondly; non-luminous gas radiation forms a significant portion

of the overall heat transfer. This component relies on high temperature water and carbon

dioxide particles radiating to the firetube inner surface. With increasing flue gas velocities

(typical of forced draft systems), the gas particle residence time, and hence the non-

luminous gas radiation in the radiant firetube section, may be reduced, however no

information on this topic could be found in the open literature.

Rather than merely guessing the first turn-around temperature, the model presented implements a

simple approach for both natural draft and forced draft. The same AGA correlation is applied to

the radiant section only and an efficiency is calculated. From this efficiency the radiant section heat

transfer rate may be calculated.

Thermal Efficiency Calculation

For natural draft immersion firetube heating applications, it is recommended that Equation 1 or

Equation 3 be used unaltered. Note, however, that the effective firetube length is longer than the

physical firetube length as explained in the section below Equation 1.

Forced draft immersion firetubes are often significantly smaller in diameter than natural draft

firetubes of similar capacity as the flow through the firetube and stack does not rely on natural

draft. Therefore, burner heat release density values and firetube velocities are typically much

higher which may also result in improved thermal efficiencies. Some manufacturers of high velocity

burners such as Eclipse’s ImmersoJet (IJ) report tested immersion firetube efficiencies that are

higher than those predicted by Equation 1. This may be due to the increased convection heat

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transfer resulting from significantly higher flue gas velocities in the firetube than what was tested

by AGA when Equation 1 was developed.

Forced draft burners are available as packaged units with the burner and combustion air fan

packaged as a single unit, as well as units requiring external fans as shown in the following figure:

Figure 7: Forced draft burners: Maxon packaged burner (left), and Eclipse burner requiring an

external combustion air fan (right).

Figure 8 below was published in the Eclipse Combustion Design Guide No 330, 10/02 (1997) for

ImmersoJet Version 2.2 Series Immersion Burners. A comparison with Figure 4 (which is based on

Equation 1) shows that the Eclipse ImmersoJet series burners offer efficiencies that are

approximately 5% higher – compare the red circle and triangle markers in Figure 4, Figure 8,

Figure 9 and Figure 10. As shown in Figure 9, the Eclipse ImmersoJet-fitted firetube produces

efficiencies of up to 5% higher than those predicted by the AGA correlation.

Figure 10 shows the published performance of the Maxon Series “67” burner. Comparison of the

triangles between Figure 10 and Figure 4 shows that the Maxon Series “67” curves are simply

based on the standard AGA correlation given in Equation 1. Note that this graph is plotted relative

to the burner heat release and not in terms of the heat transfer to the heated medium, hence

comparison with Figure 4 and Figure 8 will require the burner heat release values in Figure 10 to

be multiplied by the efficiency (75%). This graph also provides values for the maximum burner heat

release density indirectly by specifying required firetube diameters at a range of burner duties. It

can be shown that for this particular packaged burner, Maxon recommends a maximum burner

heat release density between approximately 12000 and 16500 kW/m2. Similarly, for Eclipse TFB

series burners (Eclipse Tube Firing Burners Design Guide 310), maximum burner heat release

densities between 10700 and 14000 kW/m2 are recommended. Eclipse states that exceeding these

maximum burner heat release density values may result in burner pulsation or other operational

problems.

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Figure 8: Eclipse ImmersoJet-fitted firetube efficiencies.

Figure 9: Eclipse ImmersoJet-fitted firetube efficiencies vs. the AGA correlation (Equation 1).

0

20

40

60

80

100

120

140

160

180

200

220

0.E+00 1.E+06 2.E+06 3.E+06 4.E+06 5.E+06 6.E+06 7.E+06 8.E+06

Effe

ctiv

e T

ub

e L

en

gth

[ft

]

Heat Transfer to Tank [BTU/hr]

Firetube Efficiency

AGA 85%AGA 80%AGA 75%AGA 70%AGA 65%Eclipse 85%Eclipse 80%Eclipse 75%Eclipse 70%Eclipse 65%

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Figure 10: Maxon Series “67” TUBE-O-FLAME gas burner performance (from Maxon Bulletin 2200).

In this case study, the AGA correlation is therefore adapted slightly to accommodate specific

burner manufacturer efficiency increases for forced draft burners:

(

) Equation 8

where

is the unit-dependent constant introduced in Equation 3

is the forced draft efficiency adder, typically 3% to 5%

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Flownex Immersion Firetube Model

A basic immersion firetube model has been implemented in Flownex as a compound component

which may be added to the library for reuse in future projects. This case study presents a simple

Flownex network which utilizes the firetube compound component together with previously

developed gas composition and property utility scripts, a burner compound component and a

simple natural draft stack compound component. The implementation is shown in Figure 11

below.

Figure 11: A basic immersion firetube model implemented in Flownex.

Combustion fuel may be specified in mol% directly in the Fuel Gas Supply script whilst ambient air

conditions are specified in the Combustion Air Supply script. Not only does the basic immersion

firetube model implement Equation 8, it also implements a basic burner model to perform the

combustion process associated with a firetube. As shown above, the flue gas is then ducted to a

stack which adds a natural draft component to the flue outlet. Finally, the firetube performance is

shown in terms of thermal efficiencies, heat transfer rates, heat fluxes, flue gas flow rates, velocities,

pressure losses and temperatures. Other metrics such as calculated surface areas and the

recommended minimum firetube diameter (at the burner end), a draft balance check as well as a

flue gas analysis are also presented.

The above example represents a natural draft design and that option is selected in the Basic

Firetube Input Data section. As discussed before, natural draft designs typically have much lower

firetube velocities than forced draft designs due to the limited available draft which is supplied only

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by the stack. For natural draft applications, the stack height is adjusted until the Draft Balance

Check balances. In the example above the Draft Balance Check shows that the required total

pressure at the air inlet is still higher than the actual atmospheric air pressure, and hence the

6 m-high stack is still incapable of supplying sufficient draft. This will be discussed in detail later.

Figure 12 shows the model inside the compound immersion firetube component. Combustion air

flows from the left into the burner (itself being a compound component) whilst fuel gas flows to

the burner from the top. The burner model will combust the mixture and deliver high temperature

flue gas to the radiant firetube (or radiant section). Between the radiant section and the convection

section there is the option to add a reducer for cases where the radiant section is of a larger

diameter than the convection section. Furthermore, depending on the firetube geometry – U or W

– there will be one or three 180 degree return bends.

In the interest of accuracy, the firetube radiant and convection sections are subdivided into 10

segments each. As the flue gas moves through the firetube, its properties are allowed to vary

along each segment resulting in changes in temperature, density, viscosity and velocity.

Several scripts are employed to calculate heat transfer and pressure loss coefficients. The Firetube

Efficiency Script at the top-right implements Equation 8 and also calculates the heat transfer rates

apportioned to each firetube section. These heat transfer values are then assigned to each firetube

pipe component using data transfer links which will then remove the relevant amount of heat from

the flue gas flowing through it. The same script also calculates areas, diameters and heat fluxes.

The left-most two scripts are used to obtain fuel gas heating values from the incoming fuel gas

and provide that information to the Firetube Efficiency Script. The other scripts are used to

calculate pressure loss coefficients used in the firetube flue gas pressure loss estimation.

Figure 12: Immersion firetube compound component internal elements.

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Figure 13 and Figure 14 below shows the immersion firetube compound component’s input and

resulting property pages. Note that burner heat release density and firetube heat flux warning

messages are given in the warnings area. API 12K and GPSA recommended firetube heat flux

values are also listed for convenience.

Figure 13: Immersion firetube component inputs

property page.

Figure 14: Immersion firetube component

results property page.

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Figure 15: Immersion firetube compound component internal elements.

Case Study

As an example, two equivalent immersion firetubes are sized, the first one is designed as a natural

draft system whilst the second is a forced draft system. Both will aim to achieve the same duty and

efficiency. The process requirements are set out as follows:

Single firetube application serving as a molten salt heater.

Process duty is to be 500 kW.

Target thermal efficiency is 80% (HHV).

Maximum practical self-supporting stack height is 6 m.

Site elevation is 200 m.

Design atmospheric temperature is 35°C.

Design atmospheric relative humidity is 60%.

Natural Draft Immersion Firetube Design

Natural draft designs normally use a U-shaped firetube due to the limited available natural draft

produced by the exhaust stack. From Table 2, the recommended average firetube surface heat flux

upper limit is 47 - 57 kW/m2 for a molten salt heater application. Following the API 12K

recommendation of a maximum burner heat release density of 6814 kW/m2 as discussed on

page 7, the first resulting design is shown in Figure 11 above. There are several problems with this

design:

Using the maximum practical stack height of 6 m, there is a large draft deficit as shown in

the Draft Balance Check.

The main cause of this problem is the relatively high firetube velocities of 17 m/s and

11.3 m/s.

The burner heat release density is at the upper limit.

The firetube, and hence the heater, will be quite long – approximately 12 m – which may

become impractical.

The solution is to utilize a larger firetube diameter. The redesigned immersion firetube design is

shown in Figure 16 below. It can be seen that much larger firetube diameters are required (DN450

and DN350) to achieve low enough flue gas velocities to reduce the flue gas pressure losses to

within the draft capabilities of the exhaust stack. Typical natural draft firetube velocities should be

approximately 10 m/s or less. The required exhaust stack height has been calculated as 5.4 m

which is still quite tall, moreover additional allowances for flame arrestors and possibly an exhaust

stack spark arrestor should still be made, hence the exhaust stack length could still be up to 6 m.

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Furthermore, the burner heat release density and firetube surface heat flux values are now

comfortably low for the molten salt application firetube.

Figure 16: Final natural draft immersion firetube design.

The immersion firetube is still quite long and the size (DN450) is large in comparison to other

500 kW heater applications. This may be an indication that a natural draft design may not be the

most elegant design solution for this particular application. The next section repeats the design

process using a forced draft design instead.

Forced Draft Immersion Firetube Design

Forced draft firetubes often employ W-shaped designs as the combustion air fan is sized to

produce enough pressure to overcome the additional pressure losses. For the equivalent forced

draft firetube design, the same average firetube surface heat flux upper limit of 47 - 57 kW/m2

applies, however much higher burner heat release densities can be used. Assuming a packaged

forced draft low pressure fan burner will be used as shown in Table 1, an upper limit for the burner

heat release density is taken as 15900 kW/m2.

As shown in Figure 17 below, for a forced draft design much smaller firetube diameters are

required – in fact they are almost half the nominal diameters of the natural draft design. The

results show that a W-shaped design is used with a 4.2 m radiant section, resulting in a heater of

approximately a third of the length of the natural draft equivalent. Also shown is the smaller

diameter of the convection firetube section in an attempt to keep the flue velocities approximately

constant. These velocities are based on the first turn-around temperature estimated for the

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firetube. Since the firetube radiant section is fairly short, a comparatively high first turn-around

temperature of 857°C is estimated, resulting in high turn-around velocities.

Figure 17: Final forced draft immersion firetube design.

Using a more acceptable 3 m stack height, the Draft Balance Check shows a draft deficit of 229 Pa

whilst the required combustion air flow rate is 870.9 kg/hr. This information may now be used to

select an appropriate combustion air fan. Additional allowances for fan ducting should be made

and this Flownex model could easily be extended to include these.

It is interesting to compare the two designs in terms of size and performance as shown in the

following table:

Table 3: Natural vs. Forced Draft Immersion Firetube Design Comparison.

Natural Draft Heater Forced Draft Heater

Heat transfer to process 500 kW 500 kW

Thermal efficiency (HHV) 80% 80%

Radiant section size DN450 DN250

Convection section size DN350 DN200

Approximate heater length 12 m 4.5 m

Burner heat release density 4091 kW/m2 11752 kW/m2

Firetube surface heat flux 16.8 kW/m2 40.8 kW/m2

Maximum firetube velocity 10.1 m/s 28.9 m/s

Firetube flue gas pressure drop 9.3 Pa 152.9 Pa

Stack height 5.4 m 3 m

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Even though an average firetube surface heat flux of up to 57 kW/m2 is allowable for the molten

salt application, the natural draft design only achieves a fraction of this flux due to draft limitations.

This directly contributes to the large size difference between the two designs.

Turn-Down Operation

One of the major design considerations with immersion firetubes is the influence of a possible

turn-down operation case on the performance of the firetube. As shown by Equation 8, a

reduction in the burner duty for a specific immersion firetube length will result in an increase in the

immersion firetube thermal efficiency and consequently a lower exhaust stack flue gas

temperature. For the example given in Figure 17 above, the duty has been progressively reduced

by reducing the fuel flow rate. The results are shown in Figure 18.

Figure 18: The effect of turn-down of thermal efficiency and exhaust stack temperature.

These results need to be considered in terms of:

the water dew point temperature; and

SOx acid dew point temperature.

As shown in Figure 17, the water and SOx dew point temperatures are 57.5°C and 150.8°C

respectively. The high SOx dew point temperature is due to the 1.1% (mol%) H2S present in the

fuel gas used for this example.

With efficiencies of up to approximately 87.5% (HHV) for this example, the exhaust stack

temperature is above 100°C which is still significantly above the water dew point temperature, so

the water dew point is then not a problem. However, the SOx acid dew point temperature mostly

depends on the sulfur content of the fuel gas and is typically between 120°C and 150°C. A typical

relationship between H2S content in fuel gas and the SOx (SO3 and H2SO4) dew point temperatures

are presented in Figure 19 below. Therefore as stated earlier, when suplhur is present in the fuel

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Process Duty [kW]

Effect of Turn-Down

Temperature

Efficiency (HHV)

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gas, stack temperatures should at least be 150 - 200°C minimum, and this must be considered at

the maximum turn-down (minimum duty) case.

Figure 19: The effect of fuel gas sulfur content on flue gas SOx dew point temperature.

For the above example the acid dew point temperature is 150.8°C, hence a minimum exhaust

stack temperature of 200°C should be targeted which will occur at a duty of approximately 375

kW as shown in Figure 18. Therefore, for this specific high sulfur containing fuel gas, this design

can virtually not accommodate any turn-down at all.

There are a few potential solutions if turn-down is required:

Use sulfur-free or low sulfur fuel. According to the API 12K recommendation discussed

earlier, the stack temperature can then be allowed to drop to approximately 120°C.

Reduce the immersion firetube thermal efficiency by reducing its length. This will cause a

higher stack temperature during normal operation which may be considered undesirable,

but it will also result in higher stack temperatures during turn-down, enabling the heater to

turn down further.

Employ an on-off burner control strategy instead of a modulated burner control system.

This approach is possibly the simplest and most sensible and is discussed in more detail

below.

Burner control systems typically operate in two modes: modulating and on-off. For modulating

burners the flow rate of fuel gas to the burner is modulated by an upstream flow control valve

depending on the required heat load. On-off burners are simpler and rely on the thermal inertia of

the heated fluid. The burner simply operates between maximum duty (fully on) and no duty (off)

where the burner is off and only the pilot burner remains on. Since on-off burner systems do not

modulate the burner duty, the exhaust flue gas temperature will be at the maximum value when

the burner is on and hence the stack temperature remains high. Therefore, in cases where the

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Fuel Gas H2S [mol%]

Flue Gas SO3 / H2SO4 Dew Point Temperature

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turn-down duty causes the exhaust flue gas temperature to fall below dew point values, an on-off

burner system may offer a simple solution.

Summary

A simple immersion firetube model has been developed and implemented in Flownex as a

compound component. In this case study, a natural draft and a forced draft heater were designed

to meet the same process requirements. A detailed analysis and comparison of the two designs

have been presented. The complete combustion and heat transfer process have been modeled

and the effects of turn-down operation and water and SOx dew point temperatures have been

discussed and possible problem areas highlighted. Solutions to some of the problems have also

been offered.

Flownex has the unique ability to simultaneously model combustion, heat transfer and fluid

mechanics problems. This capability makes Flownex the ideal tool to design and analyze

immersion firetube heat transfer processes. It would be much more challenging to design and

analyze an entire immersion firetube-based heater system at the level of detail presented without

Flownex as an engineering design and analysis tool.

Case Study Flownex Model Availability

The Flownex model discussed in this case study is available in the user project downloads area

located at:

http://www.flownex.com/projectlibrary